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What is the recommended method of repairing fatigue cracking and how long will the repairs last?

   

Where cracking occurs in or adjacent to welded joints, a satisfactory repair may be made by welding. It is important that the cracked material is gouged or machined away sufficiently to permit a full penetration repair weld to be made, with no traces of crack left behind and no new significant defects introduced. In theory, a good welded repair should last as long as the original joint under the same loading spectrum. However, in practice it is often found that the repair does not last as long, usually because the quality of the repair, made under conditions less favourable than those for the original fabrication, does not match the quality of the original joint. Conversely, if the cracking was premature due to the presence of significant defects in the original joint, a good quality repair has the potential to exceed the life of the original joint.

Prior to undertaking a welded repair, a full repair/welding procedure should be devised, with consideration given to the structural and metallurgical consequences of further welding. For example, there may be a risk of distortion, or a requirement for post repair heat treatment.

If the life required from the repair is greater than that achieved originally, it may be possible to obtain this by reducing the cyclic stress levels in the vicinity of the repair, and/or improving the fatigue strength. The former may be achieved by addition of material, welded or mechanically fastened to provide additional load paths, or local redesign to reduce stress concentrations. The latter can often be achieved by post-repair mechanical operations such as weld toe grinding or peening in the case of fatigue crack initiation from weld toes.

Where fatigue cracking initiates in plain material remote from any welds, a welded repair may be practical, but it will certainly have a much reduced fatigue life unless particular attention is paid to improvement of the fatigue strength and/or cyclic stress at the original crack initiation site.

In rare cases, repair may simply require that further crack extension is prevented. Drilling of holes at the crack tips, and perhaps addition of strengthening material, may be adequate. Such procedures require individual assessment to determine their effectiveness.

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